The present invention relates to systems for delivery of data over one-way broadband networks such as cable TV and satellite and, in particular, to a method for transmitting multicast data over a one-way channel of an existing broadband network infrastructure.
In a push model for distributing data, a server xe2x80x9cbroadcastsxe2x80x9d the same data to many listeners. To avoid wasting bandwidth, instead of sending the data separately to each listener, it can be sent to a xe2x80x9cmulticast groupxe2x80x9d. Anyone interested in receiving the data becomes a listener by joining the group. As the push model becomes more common, the question of what should be the underlying infrastructure, is raised.
By their nature, xe2x80x9cpush applicationsxe2x80x9d are closer to the broadcasting paradigm of radio and television than to the interactive paradigm of the World Wide Web. As such, broadband networks, such as cable TV or satellite, could be used as a very efficient medium for the transmission of xe2x80x9cpushed dataxe2x80x9d. However, these networks are one-way only, i.e. data (such as a television signal) is sent from a broadcasting facility (the head-end) to several receivers (the end-users) without any feedback from the receivers. Although attempts to upgrade the current infrastructure are underway in several places around the world, it may be years until reliable two-way broadband networks are commonplace. Therefore, an effective mechanism for multicasting over one-way broadband networks is desirable.
Commonplace TCP/IP applications, like Telnet and Web browsing, usually involve one sender and one receiver. Even in those applications where the same data is sent to several receivers by a single sender, distribution is accomplished by sending a different copy of the same data to each receiver.
IP multicasting was conceived to improve bandwidth usage in such cases. Instead of sending several copies of the same data, only one copy is sent to a xe2x80x9cgroup of receiversxe2x80x9d. Each such multicast group is assigned a class D IP address which is used as the destination address for all datagrams destined to any of the group""s members. A host that wants to receive a group joins the group by adding its address to the list of IP local addresses, and stops reception by leaving the group, i.e., removing its address from the list. Hosts join and leave groups by sending Internet Group Management Protocol (IGMP) messages to the all-hosts group. Routers listen to the all-hosts group and update their internal group membership tables according to the IGMP messages they receive.
By default, multicast datagrams are assigned a time-to-live (TTL) field of 1, i.e., they are restricted to the same subnet as the sender. However, an application is allowed to set the TTL field to a higher value so its datagrams can reach hosts beyond the local subnet. Multicast routers is keep track of joined hosts per group. If a multicast datagram with a TTL greater than 1 arrives at such a router and any host on any of the routers subnets is a member of the datagram""s destination group, then the message is forwarded (after decreasing the TTL field value) to the appropriate subnet(s).
Addresses in a specific range are intended for applications that never need to multicast more than one hop. A datagram destined for any address within the specific range is never forwarded, regardless of the TTL field value.
In the IP multicasting technique, there is no guarantee that a datagram will ever reach its destination. Consequently, if reliability is an issue, multicast applications must provide it themselves. The current approaches to error recovery are NAK-based (requesting missed data) or ACK-based (acknowledgement of each datagram received). Both approaches assume that there is a link from the multicast receivers to the multicast source.
Although hybrid systems for IP over broadband networks offer an attractive alternative for Internet access, they still suffer from some of the same problems as telephony-based systems (the need to establish a connection and extended use of the telephone line). For interactive applications there is not much that can be done to overcome these limitations until two-way broadband networks are deployed.
A main object of the invention to provide a new concept of xe2x80x9cpushingxe2x80x9d data over the existing broadband network infrastructure.
Another object of the invention is to provide a system for IP multicasting over the existing broadband networks without using any return link.
Another object of the invention is to provide a unique system for IP multicasting over a one-way channel wherein problems of multicast group membership as well as error recovery are handled locally within an end-user terminal removing any need for returning data to a host.
The invention relates to a data transmission system having a single transmitter and a plurality of receivers where the transmitter sends a group of data items to a subset of receivers over a one-way channel. Each data item is divided in blocks which are encapsulated to form datagrams, each including a block sequence number, a data item identifier, a timestamp reflecting the age of the data item. A group directory is regularly sent by the transmitter to each one of the plurality of receivers. The group directory contains information for all groups of data items enabling each receiver to select the group of data items it wants to receive.